Active bias circuit for low-noise amplifiers

ABSTRACT

A low-noise amplifier with a first amplification circuit that includes a control terminal, a first terminal, and a second terminal that communicates with a first reference voltage. An impedance load that communicates with the first terminal and a feedback circuit that comprises a current source that communicates with a second reference voltage. A comparator circuit that includes a first input, a second input and an output that communicates with the control terminal. A first impedance that communicates with the current source and the first input and that generates a predetermined reference voltage based on a reference current generated by the current source and a second impedance that communicates with the second input and the impedance load wherein the feedback circuit compares a voltage, based on an output current associated with the first terminal, with the reference voltage to generate a bias signal that is applied to the control terminal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/868,064 filed on Jun. 16, 2004 now U.S. Pat. No. 7,113,043. Thedisclosure of the above application is incorporated herein by referencein its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to integrated circuits. More particularly,the present invention relates to an active bias circuit for low-noiseamplifiers.

2. Background Information

Typically, in low-noise amplifiers (LNAs) and the like, it is desirableto reduce the contribution of noise from any component as much aspossible. In bipolar circuitry, for example, a bias current is appliedto the base of the transistor that comprises the LNA. However, due toprocess variations, temperature, and the like, the β of the transistorcan vary by a factor of two, three, four or more. As a result of suchvarying transistor characteristics, the level of the bias currentrequired to be applied to the transistor will change, resulting in avarying control of the gain of the LNA. Thus, under different conditionsand varying transistor characteristics, the gain of the LNA can vary,which in most applications is unacceptable.

To address this problem, passive circuitry can be used to control thebias current applied to the transistor. For example, a diode device canbe connected to the base electrode of the transistor. Such a diodedevice has an impedance proportional to 1/G_(m). For a 50Ω incomingsignal, such an impedance can be too low, which can result insignificant signal attenuation. In addition, any noise generated by thediode device and current sources connected to it will be transferred toand affect the rest of the LNA circuit.

Consequently, a biasing scheme is needed that is independent of processvariations, temperature, variations in the β of the transistor, andother like transistor characteristics.

SUMMARY OF THE INVENTION

A system and method are disclosed for actively controlling the bias of alow-noise amplifier (LNA). In accordance with exemplary embodiments,according to a first aspect of the present invention, a LNA includes afirst amplification circuit. The first amplification circuit includes acontrol terminal, a first terminal, and a second terminal. The secondterminal is in communication with a first reference voltage. The LNAincludes an input circuit in communication with the control terminal andan output circuit in communication with the first terminal. The LNAincludes an impedance load in communication with the output circuit andthe first terminal. The LNA also includes a feedback circuit incommunication with the control terminal and the impedance load. Thefeedback circuit includes a current source in communication with asecond reference voltage. The feedback circuit includes a comparatorcircuit. The comparator circuit includes a first input, a second inputand an output. The output is in communication with the control terminal.The feedback circuit includes a first impedance in communication withthe current source and the first input. The first impedance isconfigured to generate a predetermined reference voltage correspondingto a predetermined reference current generated by the current source.The feedback circuit also includes a second impedance in communicationwith the second input and the impedance load. According to exemplaryembodiments of the first aspect, the feedback circuit compares avoltage, corresponding to an output current associated with the firstterminal, with the predetermined reference voltage to generate a biassignal applied to the control terminal for biasing the low-noiseamplifier.

According to the first aspect, the first amplification circuit cancomprise a bi-polar transistor. The control terminal of the firstamplification circuit can comprise a base electrode, the first terminalof the first amplification circuit can comprise a collector electrode,and the second terminal of the first amplification circuit can comprisean emitter electrode. The input circuit can include, for example, acapacitive element. The capacitive element can receive an input signal.The LNA can include an isolation impedance in communication between thecontrol terminal and the output of the comparator circuit. The isolationimpedance can comprise, for example, a resistive element. The impedanceload can comprise, for example, an inductive element. The LNA caninclude a second amplification circuit in communication between theoutput circuit and the first terminal of the first amplificationcircuit. The second amplification circuit can include a controlterminal, a first terminal, and a second terminal. The first and secondamplification circuits can be arranged in a cascode configuration. Thesecond amplification circuit can comprise a bi-polar transistor. Thecontrol terminal of the second amplification circuit can comprise a baseelectrode, the first terminal of the second amplification circuit cancomprise a collector electrode, and the second terminal of the secondamplification circuit can comprise an emitter electrode.

According to the first aspect, the LNA can include a third referencevoltage in communication with the control terminal of the secondamplification circuit and each of the first and second impedances. Thefirst and second impedances can comprise, for example, resistiveelements. A value of the resistive element of the first impedance can beN times larger than a value of the resistive element of the secondimpedance. The feedback circuit can further include a third referencevoltage in communication with the second impedance, the second input,and the impedance load. The third reference voltage can include acapacitive element, and a fourth reference voltage. According to anexemplary embodiment of the first aspect, the LNA can be formed on amonolithic substrate. The LNA can be compliant with a standard selectedfrom the group consisting of I.E.E.E. 802.11, 802.11a, 802.11b, 802.11g,802.11h, 802.11i and 802.11n, or any other suitable wireless or wiredstandard.

According to a second aspect of the present invention, a LNA formed on amonolithic substrate a first amplification circuit. The firstamplification circuit includes a base electrode, a collector electrode,and an emitter electrode. The emitter electrode is in communication witha first reference voltage. The LNA includes an input circuit incommunication with the base electrode. The LNA includes a firstimpedance element in communication with the input circuit and the baseelectrode. The LNA includes a second amplification circuit incommunication with the collector electrode of the first amplificationcircuit. The second amplification circuit includes a base electrode, acollector electrode, and an emitter electrode. The first and secondamplification circuits are arranged in a cascode configuration. The LNAincludes an output circuit in communication with the collector electrodeof the second amplification circuit, and a second impedance element incommunication with the output circuit and the collector electrode of thesecond amplification circuit. The LNA also includes a feedback circuitin communication with the first and second impedance elements.

According to the second aspect, the feedback circuit includes a currentsource in communication with a second reference voltage and a comparatorcircuit. The comparator circuit includes a first input, a second inputand an output. The output is in communication with the first impedanceelement. The feedback circuit includes a third impedance element incommunication with the current source and the first input. The thirdimpedance element is configured to generate a predetermined referencevoltage corresponding to a predetermined reference current generated bythe current source. The feedback circuit includes a fourth impedanceelement in communication with the second input and the second impedanceelement. The feedback circuit includes a third reference voltage incommunication with the second and fourth impedance elements and thesecond input. The third reference voltage comprises a fifth impedanceelement and a fourth reference voltage. According to exemplaryembodiments of the second aspect, the feedback circuit compares avoltage, corresponding to an output current associated with thecollector electrode of the first amplification device, with thepredetermined reference voltage to generate a bias signal applied to thebase electrode of the first amplification device for biasing thelow-noise amplifier.

According to the second aspect, the input circuit can include acapacitive element. The capacitive element can receive an input signal.The first, third and fourth impedance elements can comprise resistiveelements. The second impedance element can comprise an inductiveelement. The fifth impedance element can comprise a capacitive element.The first and second amplification circuits can comprise bi-polartransistors. The LNA can include a fifth reference voltage incommunication with the control terminal of the second amplificationcircuit and each of the third and fourth impedance elements. Accordingto an exemplary embodiment of the second aspect, the LNA can becompliant with a standard selected from the group consisting of I.E.E.E.802.11, 802.11a, 802.11b, 802.11g, 802.11h, 802.11i and 802.11n, or anyother suitable wireless or wired standard.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will becomeapparent to those skilled in the art upon reading the following detaileddescription of preferred embodiments, in conjunction with theaccompanying drawings, wherein like reference numerals have been used todesignate like elements, and wherein:

FIG. 1 is a circuit diagram illustrating a system for activelycontrolling a bias of a low-noise amplifier (LNA), in accordance with anexemplary embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a system for activelycontrolling a bias of a LNA, in accordance with an alternative exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are directed to a systemand method for actively controlling the bias of a low-noise amplifier(LNA). According exemplary embodiments, a feedback circuit incommunication with the LNA is used. The feedback circuit senses thecurrent in the LNA. The feedback circuit includes a comparator forcomparing the sensed current with a reference current. The output of thecomparator biases the input of the LNA by adjusting the current in theLNA until the LNA current reaches a predetermined level corresponding tothe reference current. Exemplary embodiments of the present inventionprovide a means of biasing a LNA that is independent of processvariations, transistor β variations, and other like variations intransistor characteristics.

These and other aspects of the present invention will now be describedin greater detail. FIG. 1 is a circuit diagram illustrating a system 100for actively controlling a bias of a LNA, in accordance with anexemplary embodiment of the present invention. The system 100 includes afirst amplification circuit 105. The first amplification circuit 105includes a control terminal 107, a first terminal 108 and a secondterminal 109. According to an exemplary embodiment, the firstamplification circuit 105 can comprise a bi-polar transistor, such thatcontrol terminal 107 comprises a base electrode, first terminal 108comprises a collector electrode, and second terminal 109 comprises anemitter electrode. For example, first amplification circuit 105 can bearranged in a single-ended common emitter configuration such that secondterminal 109 is in communication with a reference voltage 106 (e.g., aground or any suitable reference voltage).

The system 100 includes an input circuit 110 in communication with thecontrol terminal 107 of the first amplification circuit 105. The inputcircuit 110 can comprise, for example, a capacitor 112, or any suitableimpedance element or combination of impedance elements. The system 100includes an output circuit 115 in communication with the first terminal108 of the first amplification circuit 105. The system 100 also includesan impedance load 117 in communication with the output circuit 115 andthe first terminal 108. The impedance load 117 can be any suitableimpedance element or combination of impedance elements having anyappropriate values.

The system 100 also includes a feedback circuit 120 in communicationwith the control terminal 107 and the impedance load 117. According toexemplary embodiments, the feedback circuit 120 compares a voltage,corresponding to an output current (I_(OUT)) associated with the firstterminal 108, with a predetermined reference voltage to generate a biassignal. The bias signal is applied to the control terminal 107 forbiasing the LNA.

According to exemplary embodiments, the feedback circuit 120 includes acurrent source 125, connected to a reference voltage 127 (e.g., a groundor any suitable reference voltage). The current source 125 generates awell-controlled and predetermined reference current I_(o) of anyappropriate value The feedback circuit 120 includes a comparator circuit130. The comparator circuit 130 includes a first input 132, a secondinput 133, and an output 134. The first input 132 can be, for example,the negative input of the comparator 130, while the second input 133 canbe, for example, the positive input. The output 134 of the comparator130 is in communication with the control terminal 107 of firstamplification circuit 105. The feedback circuit 120 includes animpedance 140 in communication with the current source 125 and the firstinput 132. The impedance 140 is also in communication with a referencevoltage 145, such as, for example, a DC power supply voltage (V_(DD)) orany suitable reference voltage. The impedance 140 is configured togenerate the predetermined reference voltage corresponding to thepredetermined reference current (I_(o)) generated by the current source125. The feedback circuit 120 includes an impedance 150 in communicationwith the second input 133 of comparator 130 and the impedance load 117.

According to exemplary embodiments, the output current (I_(out))associated with first terminal 108 of first amplification circuit 105produces a voltage across impedance 150. Comparator 130 compares thevoltage across impedance 140 with the voltage across impedance 150. Theoutput 134 of comparator 130 will adjust the bias current (I_(bias))applied to control terminal 107 of first amplification circuit 105 untilthe voltages across impedances 140 and 150 are equal. The impedances 140and 150 can each be, for example, resistors, such as resistors 142 and152, respectively. However, impedances 140 and 150 can be any suitableimpedance element or combination of impedance elements of anyappropriate values. According to an exemplary embodiment, the value ofresistor 142 can be N times larger than the value of resistor 152, whereN can be any number, although resistors 142 and 152 can be anyappropriate value. Thus, the bias current I_(bias) output by comparator130 will adapt to equal NI_(o).

The feedback circuit 120 includes a reference voltage 160 (e.g., an ACground or any suitable reference voltage). The reference voltage 160 isin communication with impedance 150, the second input 133 of comparator130 and the impedance load 117. The reference voltage 160 can include acapacitor 162, or any suitable impedance element or combination ofimpedance elements, and a reference voltage 163 (e.g., a ground or anysuitable reference voltage). For example, the reference voltage 160 canbe used to filter the reference voltage 145. The combination ofreference voltage 160 and impedance 150 (when comprised of, for example,resistor 152) acts as a RC filter to reduce noise injection from thereference voltage 145.

The system 100 can include an impedance 155 in communication with theinput circuit 110, the control terminal 107 of first amplificationcircuit 105, and the output 134 of comparator 130. The impedance 155 cancomprise a resistor, such as resistor 157, or any suitable impedanceelement. The resistor of impedance 155 provides isolation. For example,according to an exemplary embodiment, the resistor 157 can be 100Ω andcapacitor 112 can be 0.5 pF to match the input of the low gain mode to50Ω. However resistor 157 and capacitor 112 can be of any appropriatevalues.

FIG. 2 is a circuit diagram illustrating a system 200 for activelycontrolling a bias of a LNA, in accordance with an alternative exemplaryembodiment of the present invention. According to the alternativeexemplary embodiment, the impedance load 117 can be, for example, aninductor, such as inductor 217. However, impedance load 117 can be anysuitable impedance element or combination of impedance elements havingany appropriate values. According to the alternative exemplaryembodiment, the feedback circuit 120 will adjust the bias currentI_(bias) until the current (I_(L)) through inductor 217 reaches thedesired level (i.e., NI_(o)). The system 200 can include a secondamplification circuit 210 in communication between the output circuit115 and the first terminal 108 of first amplification circuit 105. Thesecond amplification circuit 210 includes a control terminal 212, afirst terminal 213 and a second terminal 214. According to an exemplaryembodiment, the second amplification circuit 210 can comprise a bi-polarresistor, such that control terminal 212 comprises a base electrode,first terminal 213 comprises a collector electrode, and second terminal214 comprises an emitter electrode. For example, the secondamplification circuit 210 can be arranged in a common baseconfiguration, with the control terminal 212 of second amplificationcircuit 210 in communication with reference voltage 145. The first andsecond amplification circuits 105 and 210 are arranged in a cascodeconfiguration. A cascode configuration improves stability and linearityand decreases distortion in the LNA by using the second amplificationcircuit 210 to shield the first amplification circuit 105 from voltagechanges in the system 200 by improving reverse isolation.

According to an exemplary embodiment, first and second amplificationcircuits 105 and 210 can be a n-p-n or p-n-p bi-polar junctiontransistors. However, first and second amplification circuits 105 and210 can be any suitable type of transistor, such as, for example, afield-effect transistor (FET), metal-oxide semiconductor FET (MOSFET),or the like. The reference voltage 145 for the systems 100 and 200 canbe set at, for example, approximately 3V, or any other appropriatevalue. A regulated power supply can be used for the LNA to improvesupply rejection. The input signal received on input circuit 110 can beany suitable type of electrical signal that is capable of communicatingelectrical information. The comparator 130 can be implemented using anysuitable means for performing the functions associated with thecomponent. For example, the comparator 130 can be an operationalamplifier or the like.

The components of systems 100 and 200, or any combination thereof, canbe formed on, for example, a monolithic substrate. Alternatively, eachelement, or any combination thereof, can be any suitable type ofelectrical or electronic component or device that is capable ofperforming the functions associated with the respective element.According to such an alternative exemplary embodiment, each component ordevice can be in communication with another component or device usingany appropriate type of electrical connection that is capable ofcarrying electrical information. In addition, the systems 100 and 200can be compliant with standards such as, for example, I.E.E.E. 802.11,802.11a, 802.11b, 802.11g, 802.11h, 802.11i and 802.11n, or any othersuitable wired or wireless standard.

Exemplary embodiments of the present invention can be used as at leastpart of a LNA or any other suitable type of amplifier or other circuitthat requires biasing. For example, exemplary embodiments can be used insystems for communicating information over communication channels eitherwirelessly or by wired means. However, systems 100 and 200 can be usedin any device or system that communicates information, including bothwired and wireless communication systems, read channel devices, diskdrive systems (e.g., those employing read channel devices), othermagnetic storage or recording applications, and the like, particularlywhere an amplifier circuit or the like requires a biasing signal that isindependent of process variations, transistor β variations, and otherlike variations in transistor characteristics.

It will be appreciated by those of ordinary skill in the art that thepresent invention can be embodied in various specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresently disclosed embodiments are considered in all respects to beillustrative and not restrictive. The scope of the invention isindicated by the appended claims, rather than the foregoing description,and all changes that come within the meaning and range of equivalencethereof are intended to be embraced.

All United States patents and applications, foreign patents, andpublications discussed above are hereby incorporated herein by referencein their entireties.

1. A low-noise amplifier, comprising: a first amplification circuit thatincludes a control terminal, a first terminal, and a second terminalthat communicates with a first reference voltage; an impedance load thatcommunicates with the first terminal; and a feedback circuit thatcomprises: a current source that communicates with a second referencevoltage; a comparator circuit that includes a first input, a secondinput and an output that communicates with the control terminal; a firstimpedance that communicates with the current source and the first inputand that generates a predetermined reference voltage based on areference current generated by the current source; and a secondimpedance that communicates with the second input and the impedanceload, wherein the feedback circuit compares a voltage, based on anoutput current associated with the first terminal, with the referencevoltage to generate a bias signal that is applied to the controlterminal.
 2. The low noise amplifier of claim 1 further comprising: aninput circuit that communicates with the control terminal; and an outputcircuit that communicates with the first terminal and the firstimpedance load.
 3. The low-noise amplifier of claim 1 wherein the firstamplification circuit comprises a bi-polar transistor, wherein thecontrol terminal of the first amplification circuit comprises a baseelectrode, wherein the first terminal of the first amplification circuitcomprises a collector electrode, and wherein the second terminal of thefirst amplification circuit comprises an emitter electrode.
 4. Thelow-noise amplifier of claim 2 wherein the input circuit comprises acapacitive element that receives an input signal.
 5. The low-noiseamplifier of claim 1 further comprising an isolation impedance incommunication between the control terminal and the output of thecomparator circuit.
 6. The low-noise amplifier of claim 5 wherein theisolation impedance comprises a resistive element.
 7. The low-noiseamplifier of claim 1 wherein the impedance load comprises an inductiveelement.
 8. The low-noise amplifier of claim 2 comprising: a secondamplification circuit that communicates with the output circuit and thefirst terminal of the first amplification circuit, wherein the secondamplification circuit includes a control terminal, a first terminal, anda second terminal, and wherein the first and second amplificationcircuits are arranged in a cascode configuration.
 9. The low-noiseamplifier of claim 8 wherein the second amplification circuit comprisesa bi-polar transistor, wherein the control terminal of the secondamplification circuit comprises a base electrode, wherein the firstterminal of the second amplification circuit comprises a collectorelectrode, and wherein the second terminal of the second amplificationcircuit comprises an emitter electrode.
 10. The low-noise amplifier ofclaim 9 further comprising: a third reference voltage that communicateswith the control terminal of the second amplification circuit and eachof the first and second impedances.
 11. The low-noise amplifier of claim1 wherein the first and second impedances comprise resistive elements.12. The low-noise amplifier of claim 11 wherein a value of the resistiveelement of the first impedance is N times larger than a value of theresistive element of the second impedance.
 13. The low-noise amplifierof claim 1 wherein the feedback circuit further comprises: a thirdreference voltage that communicates with the second impedance, thesecond input, and the impedance load.
 14. The low-noise amplifier ofclaim 13 wherein the third reference voltage comprises: a capacitiveelement; and a fourth reference voltage.
 15. The low-noise amplifier ofclaim 1 wherein the low-noise amplifier is formed on a monolithicsubstrate.
 16. The low-noise amplifier of claim 1 wherein the low-noiseamplifier is compliant with a standard selected from the groupconsisting of I.E.E.E. 802.11, 802.11a, 802.11b, 802.11g, 802.11h,802.11i and 802.11n.